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The Evolution engine (popularly known as Evo) is an air-cooled, 45-degree, V-twin engine manufactured from 1984 by Harley-Davidson for the company's motorcycles. It was made in the displacement for Harley Davidson Big V-twins bikes, replacing the Shovelhead engine until 2000 when the last EVO was placed in a production factory custom FXR4 (FXR2 and FXR3 were the first CVO's). In 1999, it was replaced by the Harley-Davidson Twin Cam 88 in the Touring and Dyna model and in 2000 in the Softail models. Also available in the Sportster model beginning in 1986, it was made in the displacement until 1988 and is still made in the and displacements for the Harley-Davidson Sportster, replacing the ironhead Sportster engine. Most analysts consider the Evolution to be the engine that saved the reorganized Harley-Davidson company from certain bankruptcy. Harley-Davidson's official name for the engine was likely related to the company's attempt to reform its image following the 1981 management buyout from previous owner American Machine and Foundry (AMF).〔''Smithsonian'' magazine, August 2003, pg. 36 – "Wild Thing", Robert F. Howe〕 Though a major design advance for Harley-Davidson in many ways, the Evolution is most distinct from earlier Harley-Davidson engine designs by virtue of its reliability, oil tightness, and ability to be run hard under all kinds of circumstances for tens of thousands of miles farther than any of its predecessors. Both the heads and cylinders of the Evolution engine are made from aluminum to reduce weight compared to a cast iron design. Air cooling efficiency is improved as aluminum is a superior thermal conductor to cast iron. The use of aluminum for both heads and block reduces wear on the head gasket seals, compared to a combined cast iron and aluminum design which could induce stress on the head gasket seals as a result of different rates of expansion and contraction between dissimilar metals.〔(Accubilt Online )〕 The blocky rocker boxes (thus becoming nicknamed "block head" which never caught on), aluminum heads and cylinders (also referred to as "jugs") are the only part of the Evolution engine that can be said to be essential; the Big Twin and Sportster incarnations of the Evolution are significantly different. ==Evolution on the Sportster== The unit construction of the Harley-Davidson Sportster, which has been part and parcel with the highly successful model line since its inception as the sidevalve 750cc "K" Model in 1952, was retained with the Evolution engine upgrade in 1986, resulting in a unique valve train configuration. Unlike almost any other engine in production today, the Sportster Evolution uses one cam per engine overhead valve, resulting in four individual, single-lobe, gear-driven camshafts. The cam lobes are thus all located one behind another, and pushrods are arrayed in pairs (front and rear) parallel to the cylinder axis as a result. This allows each lifter and pushrod to deflect from the cam lobes perpendicular to the lobe plane. This configuration is friendly to radical, high-output cams, making the Sportster Evolution a natural choice for the once Harley-Davidson owned line of Buell Motorcycle Company sportbikes from 1986 up to late 2009 (2010 model year). The Sportster Evolution engine has remained largely unchanged from 1986 to 2006, though changes to the transmission, final drive and motor mounts have necessitated changes to the Sportster Evolution case. Construction is almost entirely the same between 883 cc and 1,200 cc versions; the chief difference between the two is a smaller bore on the 883 cc (inherited from its earlier "ironhead" parent), along with slightly different heads. Conversions from 883 cc to 1,200 cc are relatively inexpensive and commonplace, and cheaper than the price premium to go from an 883 to 1,200 engine on a new bike. Carburetors were standard on Sportster engines until 2007, when they were replaced by the Delphi Electronic Sequential-Port Fuel Injection (ESPFI) system. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Harley-Davidson Evolution engine」の詳細全文を読む スポンサード リンク
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